Abstract
Flash vacuum pyrolysis (FVP) of 3-[2-(t-butylthio)phenyl]propenoate 7 at 700 °C (0.01 Torr) unexpectedly gave a mixture of benzo[b]thiophene derivatives 5 (15%) and 8 (21%), and thiocoumarin 9 (28%). Control experiments show that thiophenoxyls (e.g. 15) cyclise efficiently to benzo[b]thiophene 5 under similar conditions. It follows that FVP of S-t-butyl derivatives of thiophenols, is not an efficient means of generating thiophenoxyl radicals owing to competing hydrogen capture processes.
Highlights
We have shown that benzo[b]furans 4 can be generated by flash vacuum pyrolysis (FVP) of 2(benzyloxy)- or 2-(allyloxy)-cinnamate esters 3 (X = O); these precursors act as sources of the corresponding phenoxyl radical by homolysis of the weak O-benzyl and O-allyl bonds respectively (Scheme 1).[1,2]
In order to probe the effectiveness of S-t-butyl groups as gas-phase thiophenoxyl radical generators, the cinnamate 7 was readily made under standard Wittig conditions from the commercially available aldehyde 6
It is known that FVP of 2-hydroxycinnamates is a good synthetic route to coumarins[14] so we propose that the thiocoumarin 9 is formed from the thiophenol 19 by an analogous mechanism (Scheme 5)
Summary
We have shown that pyrolysis of S-t-butyl derivatives is a poor route to thiophenoxyl radicals under FVP conditions, probably because an increased hydrogen atom flux compared with allyl and (especially) benzyl precursors allows a number of other pathways to compete with cyclisation. After dilution with water (100 cm3) the product was filtered, washed with ether (3 × 10 cm3) and recrystallised from ethanol to provide 2-(benzylthio)benzoic acid (11.61 g, 71%) m.p. 190-191 °C (lit.,13 188-189 °C); δH 10.70 (1H, s, OH), 8.05-7.30 (9H, m) and 4.23 (2H, s).
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